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Direct injection (DI) of compressed natural gas (CNG) is a promising technology to increase the indicated thermal efficiency of internal combustion engines (ICE) while reducing exhaust emissions and using a relatively low-cost fuel. However, design and analysis of DI-CNG engines are challenging because supersonic gas jet emerging from the DI injector results in a very complex in-cylinder flow field containing shocks and discontinuities affecting the fuel-air mixing. In this article, numerical simulations are used supported by validation to investigate the direct gas injection and its influence on the flow field and mixing in an optically accessible ICE. The simulation approach involves computation of the in-nozzle flow with highly accurate Large-Eddy Simulations, which are then used to obtain a mapped boundary condition. The boundary condition is applied in Unsteady Reynolds Averaged Navier-Stokes simulations of the engine to investigate the in-cylinder velocity and mixing fields. The velocity field has been measured using time-resolved stereoscopic Particle-Image Velocimetry in a running engine. The scalar field indicating the injected gas has been measured with Holographic Tomographic Interferometry. The cycle-to-cycle fluctuations in the measured velocity field are found to be high, presumably due to shocks and their reflections from cylinder walls. The proposed simulation approach can reasonably predict the ensemble-averaged velocity field. Yet, it cannot predict the wall-attached flow of the gas jet observed in the scalar field measurements. The impact of the gas jet from a centrally mounted injector on tumble motion and turbulent kinetic energy has been investigated. Gas injection at high injection pressures and lower engine speeds destroys the tumble flow generating high turbulence levels, which benefits the mixing. However, as a result, overall turbulence levels are reduced near the combustion top dead center compared to those without injection, which may lead to slower combustion in a fired engine.